1. Field of the Invention
The present invention relates to a material for use in diffusion transfer photography. More particularly, the present invention relates to a silver salt diffusion transfer photographic film unit wherein a light-sensitive element is exposed in a camera and developed, and which functions to provide the appearance of the image in a bright place outside the camera.
2. Description of the Prior Art
The so-called silver salt diffusion transfer photography wherein exposed silver halide grains are developed and then the remaining unexposed silver halide grains are dissolved in the form of a silver complex salt and diffused into a separate image-receiving layer, whereby a transferred positive image of the silver is obtained by the action of silver precipiting nuclei present in the image-receiving layer, has features that a high sensitivity is obtained and a positive image is directly obtained at a place separated from the negatively developed silver in a single developing processing. This process is widely used in materials for office copy and highly sensitive materials for photography.
A film unit wherein a photosensitive element is exposed in a camera, brought in contact with a processing composition, withdrawn from the camera while maintaining the photosensitive element in a light-shielded condition, and developed in a bright place, and furthermore the production of the transferred silver image can be successively observed in a bright place and stored without separating the image-receiving layer, is useful. For instance, in the field of color transfer photography, with the film units as described in U.S. Pat. Nos. 3,415,644, 3,573,043, and 3,613,421, an image is produced on the side to which an imagewise exposure is applied. In this case, an optical system for the reversal of the image must be incorporated into the camera for the purpose of preventing the production of an image wherein left and right are reversed. Thus, the construction of the camera is specific and complicated, and special attention should be paid for maintenance of the accuracy of focus.
A film unit wherein a transferred image is produced on a side opposite that to which the exposure is applied is advantageous in that it can be used in commonly used cameras. In this type of film unit, however, both sides of the photosensitive element should be shielded against light during the processing. The side opposite that to which the exposure is applied can be shielded by coating a layer which is permeable to a processing solution containing a hygroscopic material such as carbon, whereas the side to be imagewise exposed should be provided with a means for completely shielding the element from strong light during the processing in a bright place although it contains no material which scatters and absorbs light during the processing. As the light-shielding means for the surface to be exposed during the processing, a light-shielding element which is hinged at one end of the film unit, as described in U.S. Pat. Nos. 3,415,645 and 3,415,646, is known. This element is removed from the surface to be exposed during the imagewise exposure and it covers an exposed area when the processing solution is spread. In removing, however, the light-shielding element in such manner in the camera, many problems arise in that a large space is required in the camera, the mechanism of the camera is complicated, and the construction of a cassette for accommodating the film unit is complicated, etc. Another light-shielding means used during the processing of the exposed area of the film unit, as described in U.S. Pat. No. 3,635,707, involves spreading a hygroscopic material containing the processing solution on the exposed area of the photosensitive element. This method, however, can be applied to a film wherein an image-receiving layer and a photosensitive layer are superposed on a support while it cannot be applied to the layer construction of the present invention wherein the image-receiving element and the photosensitive element are coated on different transparent supports.